This invention relates to the field of composite membranes, particularly those that comprise microporous polyacrylonitrile ("PAN") fiber or film having a selective layer comprising 4-polyvinylpyridine ("PVP") or the polyimide that is the polymerization product of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 3,5-diaminobenzonitrile] ("6FDA-DBN").
In creating a gas or liquid separation membrane, it is desirable to have both a high rate of permeation or throughput and a high separation factor. (The separation factor of two components to be separated is defined as the ratio of the permeance of the component that preferentially passes through the separation membrane to the permeance of other component.) This combination of characteristics permits the effective separation of a relatively large volume of fluid per unit time. Large rates of permeation also allow the membrane to be operated at lower pressures, which improves cost-effectiveness.
Highly porous membranes tend to be very permeable, but perform poorly at separating fluids into their components. Less porous, dense membranes can be more selective, but at the cost of reduced throughput.
Asymmetric membranes generally have a thin, dense separation or selective layer and a less dense, more permeable layer. These asymmetric membranes are often chosen for separation applications because they may provide a good combination of permeation and separation. Hollow fibers having a dense skin and a porous body can be used to make such membranes.
Polyacrylonitrile hollow fiber may be coated with another material that serves as a selective layer to form a useful separation membrane, but the throughput of the PAN composite membrane may be too low for practical purposes.
U.S. Pat. No. 3,851,036, issued to Tzentis, describes a method for making asymmetric hollow fibers comprising acrylonitrile polymers, which fibers may be used in dialysis cells or textiles.
A process for producing hollow polyacrylonitrile separation membranes in fibrous form is described in U.S. Pat. No. 4,409,162, issued to Takeo. In this process, a membranous product is shaped in hollow fiber form from a solution of PAN and subsequently removing solvent to produce an asymmetric porous separation membrane.
U.S. Pat. No. 4,756,932 issued to Puri describes a process for making highly permeable coated composite hollow fiber membranes. The hollow fiber substrate is passed continuously through a polymeric coating solution and withdrawn through a coating die to form a uniform coating.
U.S. Pat. No. 4,881,954 issued to Bikson, et al. describes permeable membranes for enhanced gas separation in which an asymmetric porous support is coated with a separation layer. The support is made asymmetric prior to coating to provide mechanical stability and a uniform support surface.
U.S. Pat. No. 4,935,141, issued to Buck, et al., describes asymmetric composite separation membranes for use in hemodialysis. These membranes may comprise PAN.
U.S. Pat. No. 4,960,519, issued to Pasternak, et al., describes a membrane process for separation of organic liquids using a composite membrane in which polyacrylonitrile is the support layer.
U.S. Pat. application 739,063 by Chung, et al., filed on Aug. 1, 1991 and entitled "HIGH SELECTIVITY HOLLOW FIBERS", describes asymmetric hollow fibers comprising SIXEF.TM.-Durene polyimide coated with PVP. The disclosure of this application is incorporated herein by reference.